1. Field of the Invention
The invention relates to a frequency generator, more particularly to a digitally controlled frequency generator.
2. Description of the Related Art
Referring to FIG. 1, a conventional frequency generator is shown to include a crystal oscillator 9 for generating a clock signal, an input module 81, and a control module 82 coupled to the crystal oscillator 9 and the input module 81. The crystal oscillator 9 includes a resonance chip 90 and a tuning circuit 91. The resonance chip 90 is fabricated through cutting, waxing, halfing, rounding, dewaxing and frequency lapping procedures. The tuning circuit 91 includes an inverter 911 and a plurality of variable load capacitors (C1, C2, C3). The input module 81 is operable so as to output a control signal to the control module 82. The control module 82, such as a micro-controller, controls the crystal oscillator 9 in response to the control signal from the input module 81 to adjust capacitances of the load capacitors (C1, C2, C3) so that the clock signal generated by the crystal oscillator 9 has a specified resonance frequency.
However, the frequency of the clock signal provided by the conventional frequency generator is fixed and cannot be adjusted by the user to obtain a desired frequency.